A thermal dispersion switch can be used for sensing either flow or level. A sensor includes two probes disposed in a process media. The operation is based on the dispersion of thermal energy by the process media as it flows past the probes, or comes into contact with the probes. Air and other gases generally have less heat absorbing capacity than liquids. When the media is flowing the process acts to move the material away from the sensor probes so that the thermal absorbing capacity is greater under flow conditions. In stagnant, i.e., no flow, conditions, the sensor probes cause the media around them to stabilize at some temperature higher than the typical process temperature. A thermal dispersion switch uses these factors to test for either the presence of air/gas or liquid with a level switch, or the rate of flow being above or below a specifically set flow rate.
In one known form, the thermal dispersion switch includes a sensor having two probes each containing a resistance temperature device (RTD). The resistance of the RTD varies with temperature. The first RTD is internally heated to establish a temperature above the process temperature. The second RTD is unheated and provides reference temperature of the process so that the measurement can be compensated for the process temperature. The first RTD senses the changes in thermal dissipation of the flow and/or liquid level. A sensing circuit responds to this information to provide an output generally in the form of a relay contact closure.
Advantageously, a process instrument should be periodically tested to verify proper operation of the instrument circuitry. Performance of such tests required that the instrument be removed from its application. This usually entailed disconnecting electrical terminations and conduit and other appurtenances. Not only was such a procedure time consuming, it might also require process downtime.
An additional problem results during start-up of a thermal dispersion switch, as time is required for the first probe to be heated to a sufficient level for proper operation. Prior to heating of the probe, the instrument would sense a high flow or level condition, thus actuating an alarm or other control circuits.
The present invention is directed to overcome one or more of the problems discussed above in a novel and simple manner.